We propose to continue our research into the genetic basis of dietary obesity in mice. With the ever- expanding pandemic of obesity it has become apparent that some individuals will add fat on a high fat diet while others are seemingly protected from this effect. Some of this variation in dietary obesity is due to variation in genetic factors. We have identified 4 fat depot QTLs where genotypes respond differently to a high fat diet in our genetic mapping studies using the LGXSM Recombinant Inbred (Rl) mouse lines. In the first specific aim, we will follow-up our earlier experiments by fine mapping and endeavoring to validate quantitative trait loci that differ in their effects when challenged by a low or high fat diet. We have already generated and measured a large variety of traits in over 1000 animals from the LG/J by SM/J Advanced Intercross (Al) Line. This population allows 8 times the mapping resolution of a F2 intercross population. We will genotype these Al animals for molecular markers covering the four dietary obesity QTLs mentioned above reducing them to a sub-cM region. This mapping will allow us to identify a limited number (10-15) positional candidate genes using information from the mouse genome database.Under the second aim, the positional candidate genes at each QTL will then be tested for exonic sequence differences between LG/J and SM/J. Any sequence differences discovered will be evaluated for potential functional significance using bioinformatics and comparative genomic approaches. Potential functionally significant polymorphisms will be tested in appropriate functional assays. Under the third aim we will assay animals from the LG/J and SM/J strains reared on high and low fat diets for differences in the patterns and levels of gene expression for our positional candidate genes using qPCR. Significant results will be followed by mapping expression polymorphisms in the LGXSM Rl strains. Expression traits that map to the QTL region indicate cis-regulatory variation that may be responsible for the observed QTL effect. These studies will increase our knowledge of the important physiological effects of dietary fat, including the genetic variants responsible for individual variations in response to a high fat diet. [unreadable] [unreadable] [unreadable]